Magnetostriction measurements on single crystal and polycrystal TbFe2 and ErFe2 are reported and compared. For TbFe2 we find the largest known room temperature magnetostriction constant: λ111=2400×10− 6. For ErFe2: λ111=−300×10− 6. λ111 is the dominant magnetostriction constant and the major source of the magnetostriction of the polycrystalline rare earth‐Fe2 alloys. TbFe2 possesses the largest known cubic anisotropy constant. At room temperature K1=−7.6×107 erg/cm3. The sign of the anisotropy plays an important role in determining the field dependence of the polycrystalline magnetostriction.